It has long been the practice with those directly interested in undertaking a physical fitness or body building program to utilize the conventional weight lifting devices, such as the common barbell or dumbbell training sets, for the purpose of systematically performing various isotonic exercises which provide for a toning of selective muscles throughout the body. That such exercises are desirable for individuals of all backgrounds and status is readily apparent from the current interest and enthusiasm that has been generated by both the medical profession and physical fitness advocates who urgently recommend that all members of the general public discipline themselves to a form of systematic and individually oriented physical fitness program.
In performing body firming exercises, it is necessary, to achieve maximum effect, that the selective skeletal muscle or muscles toward which the physical exertion is directed not only be tightened and hardened during the exercise, but should likewise concurrently be alternately contracted and extended to achieve maximum toning and development. Muscle construction is basically a formation of tissue which is fibrous in content, and to simply tighten this fibrous tissue in performance of an exercise without stretching or contracting the muscle fails to provide its full development. For this reason, the so-called isometric exercises which are designed to acquire body toning by stationarily pitting ones muscular strength against an immovable object fails to achieve maximum effect. Also, other body exercises performed during calisthenics do provide selective body movement and accompanying expansion and contraction of muscle tissue, but fail to acquire maximum efficiency in muscle development because such exercises do not incorporate the use of supplemental resistance to concurrently force exertion of the muscle to its maximum endurance. The most effective form of body building exercises combines selective body movements directed towards one or more muscles while incorporating the use of extrinsic resistance to force the muscle to function under pressure. For this reason, exercises performed while using the common weight lifting devices are very effective for achieving body development to its fullest and resulting physical fitness.
Various problems are readily manifested to those exercising with the common form of weight lifting devices that are presently available upon the market. To undertake a complete program of physical fitness with the use of a standard set of free weights ordinarily requires the acquisition of hundreds of pounds of various weighted plates and other accessories requiring large amounts of floor space. Such weight lifting sets are difficult to transport, because of the many loose components of excessive weight. Furthermore, the use of free weights can be dangerous and should not be undertaken without a training partner or assistant.
Another deficiency of the common weight lifting devices is the absence of a means for controlling the force experienced by the user during the course of a given exercising stroke. Due to the biomechanics of the human body, the leverage exerted by the muscles on the bones varies with the position of the bone being acted on. Typically, a muscle is at its weakest leverage point when fully extended, with a general increase occurring as contraction progresses. This is most obvious in the long bones. In an arm or leg press, for example, at the start of the stroke while the elbows or the knees are bent to the limit the exercised muscles are fully extended, and there is a minimum driving capability. Then, as the stroke progresses with the arms or legs extending to a greater degree, the driving capability increases. The same is true for a curling exercise. With the arms fully extended, the mechanical advantage of the muscle and joint configuration is very low in terms of overcoming a force that resists the bending of the elbows. Then, as the stroke proceeds with the elbows bending more and more, the mechanical advantage increases. It is, therefore, desirable in the cases discussed, that the force produced by the exercising device should be relatively low at the initiation of an exercising stroke and should increase as the stroke progresses. Furthermore, the rate at which the force increases with displacement should also be controllable. Ideally, such a controlled force pattern should conform with body strength as it varies with position, so that the apparent resistive force remains constant throughout a given exercising stroke.